The invention relates to electronic devices for generating a current and voltage signal.
In industrial instrumentation, electric drives, such as motor drives and frequency converters, and other drives, analog outputs are often needed to signal various matters related to the operation of the drive. These include control and regulating signals or measuring signals. A 4 to 20-mA current loop is one analog electric communication standard that is generally used in this type of analog signaling. In a 4 to 20-mA current loop a 4-mA loop current typically represents a 0% signal value and 20 mA represents a 100% signal value. Correspondingly, a voltage signal, such as a 0 to 10-VDC voltage signal, can be used in analog signaling. In instrumentation, a device generating and transmitting an analog signal is generally called as a transmitter. Current loops in particular are often used to control separate analog panel gauges due to their easy readability.
Conventionally used current loops have several drawbacks that in certain cases may even lead to dangerous situations. A traditional simple current loop transmitter is often implemented by a pulse width modulation (PWM) principle, in which a signal coming from a controlling microprocessor, for example, switches a reference voltage on and off at a suitable pulse ratio and the resulting rectangular wave is filtered into a voltage command to a separate analog current generator. If high accuracy or resolution is required of the current loop, a current generator command formed by the PWM technique is usually not sufficient and a D/A converter with the required accuracy and resolution can be used. In both cases, current calibration and stability are completely dependent on fixed components, whereby the temperature dependency of the components may cause unexpected errors. Thus, there is no certainty as to the actual amount of the current passing through the current loop and whether the current loop has possibly been broken, in which case the analog gauge it controls will misleadingly indicate a zero value, even though the connection may in reality be dangerous because it is live. If the scales of panel gauges have been made for bi-directional display variables, it is preferable that the zero point of the gauge is in the middle of the scale even when the loop feeding the gauge is dead. This requires a bipolar current loop, in other words, it must be possible to change the direction of the current in the loop in accordance with the indication requirement of the gauge. Conventional current transmitters are not able to do this. If high accuracy or resolution is required of the current loop, a current generator command formed by a PWM technique is usually not sufficient and a D/A converter with the required accuracy and resolution needs to be used. The temperature dependency of the actual current generator part may still bring about unexpected errors that cannot be detected at all. In addition, accurate or high-resolution D/A converters are expensive which increases the costs of the signal transmitter.